This invention relates to noise reducing circuits, and more particularly to a noise reducing circuit adapted to reduce noise contained, for example, in an FM multiplex signal, a video signal reproduced by VTR, or a pickup-reproduced signal reproduced by a CD or DVD (Digital Video Disk) player.
In a conventional noise reducing circuit shown in FIG. 43, when a luminance signal is supplied through a high-pass filter (HFP) 2 and amplified by a limiter amplifier 3, an amplified signal, that is limited at a predetermined level and the higher as shown in FIG. 44B, is obtained from the limiting amplifier 3. Meanwhile, when a luminance signal shown in FIG. 44(A) is supplied through an HFP 4 and amplified by an amplifier 5, an amplified signal is obtained that has a maximum level nearly coincident with a limiting level of the limiter 3. The subtracter 6 subtracts an output of the amplifier 5 from an output of the limiter amplifier 3 to attenuate a level thereof, thereby outputting noise as shown in FIG. 44(D) from the subtracter 6. Accordingly, it is possible to reduce the noise contained in the luminance signal by subtracting the noise shown in FIG. 4(D) from the luminance signal shown in FIG. 44(A) with using the subtracter 6.
In this prior art, however, if a broad-band luminance signal is inputted, an extraction signal having a luminance signal component superposed with noise is obtained from the HPF 2. There has been a problem that noise is saturated by the limiter 3 with the result that no sufficient noise can be obtained from the subtracter 6.
Incidentally, there is another example of a noise circuit of this kind disclosed in Japanese Laying-open Patent Publication No. H2-239778 [H04N 5/21, G11P 20/02, H03H 15/00] that has been laid open on Sep. 21, 1990. According to FIG. 1 of this publication, when one or both of two switches is turned on, noise fallen on a passband of one or both of filters is supplied to a subtracter to thereby reduce noise from a reproduced signal. In such a noise removing filter, however, where any one of the two switches only is turned on, there arises, similarly to the above case, a problem that there is a saturation of noise superposed on a signal extracted from a corresponding filter. This problem still remains even where the both switches are turned on.
Therefore, it is a primary object of this invention to provide a noise reducing circuit that is capable of reducing noise contained in an input signal.
A first invention is a noise reducing circuit, comprising: a first signal path for receiving an input signal and outputting a noise component contained in the inputted signal; a second signal path for receiving the inputted signal and outputting the input signal or a first correlating signal correlating to the input signal; and a first calculating means for calculating the noise component outputted from the first signal path and the input signal or the first correlating signal outputted from the second signal path, and removing noise; wherein the first signal path includes a plurality of bandpass filters for extracting a plurality of first predetermined frequency components contained in the input signal, a plurality of limiters for individually receiving outputs of the bandpass filters, and a first creating means for creating the noise component based on outputs of the limiters.
A second invention is a noise reducing circuit, comprising a plurality of bandpass filters for extracting predetermined frequency components contained in an input signal; a plurality of amplifiers having a range in which a gain becomes zero or the slight with respect to an input level and having the predetermined frequency components; and an adding means for adding together input signal components contained in outputs of the amplifiers.
In the first invention, the noise component is outputted from the first signal path while the input signal or the first correlating signal correlating to the input signal is outputted from the second signal path. The first calculating means calculates the noise component and the first correlating signal to reduce the noise. In the first signal path, the bandpass filters divide the input signal into a plurality of bands to extract the predetermined frequency components. These first predetermined frequency components are individually given to the limiters where they are limited at a level higher than a predetermined level, i.e. the input signal is limited at a level higher than the predetermined level. The first creating means reduces, for example, input signal components contained in the outputs of the limiters to output a noise component.
In the second invention, the input signal is divided by the bandpass filters into a plurality of bands to extract the predetermined frequency components. These predetermined frequency components are individually given to amplifiers having a range of a gain of zero or the slight. Due to this, the noise contained in the predetermined frequency components is reduced. The outputs of the amplifiers are added together by the adding means, thereby obtaining an input signal reduced of the noise.
According to the first invention, since the input signal is divided by the bandpass filters into a plurality of bands, the noise contained in the input signal can be fully reduced without saturating the noise superposed on the extracted signal from the bandpass filter by the limiter.
According to the second invention, since the input signal is divided into a plurality of bands by the bandpass filters, it is possible to obtain the input signal fully reduced of the noise without amplification of the noise by the amplifier.
The above described objects and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.